Coffee grinders are known, comprising a casing housing two superposed grinding wheels, one of which is rotated by an electric motor, the other being movable axially relative to the former by operating a threaded ring nut engaging in the casing which houses the grinding wheels.
In these machines the degree of grinding is continuously adjustable by the threaded coupling, which enables the grinding wheels to be moved axially towards and away from each other, and by a system of elastic members which enable any slack arising within the connection between the constituent mechanical members of the adjustment system to be taken up.
These known adjustment systems present however the drawback that the set degree of adjustment is not always guaranteed, as the vibrations caused by the motor, the expansion of the system components caused by temperature gradients, and the thread wear caused by the passage of time and by use, cause the ring nut to move relative to the casing, with consequent variation in the degree of grinding.
To obviate these drawbacks, locking systems have been proposed consisting of a screw which causes the ring nut to engage the casing or the grinding wheel carrier.
However, if the screw engages the casing, the slack in the thread means that the ring nut loses perpendicularity to the motor axis, with consequent loss of perpendicularity to the grinding wheel carrier.
If the screw engages the grinding wheel carrier, the traction exerted by the screw causes loss of perpendicularity to the grinding wheel carrier, with consequent grinding irregularity.
Locking systems ensuring grinding wheel parallelism also exist, however these systems do not enable the degree of grinding to be adjusted continuously.